In a conventional flow measurement device, a flow sensor is attached on the upper part of the wall surface of a flow path formed in a body. On the upstream side of the flow sensor, a plurality of straightening meshes is disposed via spacers at constant intervals. On the downstream side of the flow sensor, a mesh is also disposed. The flow sensor is disposed so that the detector plane of the flow sensor faces the flow path. The flow sensor calculates a flow rate based on a voltage corresponding to a flow velocity of the fluid to be measured and outputs it.
It is ideal that the fluid flowing in the flow path of the flow measurement device presents a uniform flow (steady flow) in which direction and magnitude of flow are same at any positions in the cross section of the flow path. However, the real fluid does not always present such a steady flow and has different direction and magnitude of flow at each position of the cross section of the flow path, causing drift or involving vortex (unsteady flow). This is caused by a wide variety of factors, for example, bend of piping connected to the upstream side and down stream side of the flow measurement device, pulsation and deviated velocity distribution due to a discharged condition of a pump, viscosity and density of the fluid and so on.
In order to reduce disturbance of the fluid, it is proposed in Japanese publication of examined application No. H6-43907 to provide a reduced portion in a flow path backward of a straightening metal mesh. The flow measurement device provided with such reduced section will be explained hereinafter in accordance with a schematic view of FIG. 22. On the upstream side of a flow path 101, a plurality of straightening meshes 103 is disposed via spacers 102. Backward of the meshes 103, a reduced portion 105 is formed toward a flow path 101 in which a flow sensor 104 is provided. On the down stream side of the flow sensor 104 is formed an expanded portion 106. In the expanded portion a mesh 107 is disposed.
However, even in the flow measurement of the above patent document in which the plurality of straightening meshes 103 and the reduced portion 105 are provided, their functions are not sufficiently performed and the flow is not perfectly straightened. For example, when a fluid with a disturbed flow distribution which is caused by, for example, a bend shown by an arrow 107 existing in a piping connected to the upstream side of the flow measurement device enters the device, the disturbance could not be suppressed by the straightening meshes 103 and the reduced portion 105. Thus, the disturbance has been continued to the downstream side so that at the position of the flow sensor 104 the flow in the upper portion of the flow path has become slower and the flow in the lower portion has become faster. As a result, the flow sensor 104 has output a smaller flow rate than a mean flow rate.